1. Field of the Invention
This invention relates to coating compositions for the formation of a back coat layer in a magnetic recording medium. More specifically, this invention relates to magnetic recording mediums excellent in antistatic properties, anti-irregular winding ability, abrasion resistance, running stability and the like and having lower dynamic friction.
2. Description of the Related Art
Magnetic recording mediums such as magnetic tapes have heretofore been used widely. They are required to have a high recording density. In addition, they are required to have excellent antistatic properties, anti-irregular winding properties, abrasion resistance, running stabilizing ability and like properties and have lower dynamic friction upon both production and use, because they are supplied in a rolled form but in use, they are brought into contact at magnetic layers thereof with a magnetic head and at back sides thereof with parts of a tape drive mechanism while being caused to run at a high speed. It is known that to meet such requirements, a back coat layer with fillers, including carbon black, dispersed and bound in and by a binder is provided on the back side of a non-magnetic base film, thereby providing a magnetic recording medium with improved antistatic properties and abrasion resistance.
The following methods have been proposed for the formation of such a back coat layer as described above.
(1) To use carbon blacks of different particle sizes in combination [Japanese Patent Application Laid-Open (Kokai) No. SHO 59-14127].
(2) To provide a back coat layer with a lubricant layer composed of an organometallic compound [Japanese Patent Application Laid-Open (Kokai) No. HEI 2-214015].
(3) To use a polyurethane resin containing tertiary amino groups in its molecule as a binder for a back coat layer [Japanese Patent Application Laid-Open (Kokai) No. HEI 1-92317].
Representative properties which are required for such back coat layers are to improve the antistatic properties, abrasion resistance and lubricity and also to reduce the dynamic friction coefficient.
To impart such properties to a back coat layer, it is principally required to stabilize the dispersion of carbon black in a coating composition, to provide the back coat layer with stabilized gloss and also to provide the back coat layer with high adhesion to a non-magnetic base film. The prior art however failed to sufficiently meet the principal requirements.
Addition of a polyisocyanate curing agent to a coating composition for further improvement of physical properties of the resulting back coat layer as a film and its adhesion to a non-magnetic base film deteriorates the dispersion stability of carbon black in the back coat layer and also the gloss of the back coat layer. The back coat layer with inferior gloss has a rough surface and is therefore accompanied by the problem of inferior abrasion resistance.